The present invention relates to hopper dredges, whether tug or self-propelled. A hopper dredge is a vessel carrying equipment for removing material from the submarine bottom in order to harvest the material for use or to deepen the waterway and form a navigation channel therein or the like. The material removed from the submarine bottom is deposited in hoppers on the vessel and, when the hoppers are full, the vessel transports the dredged material to a predetermined location for off-loading by pumping it overboard or by dumping the hopper contents through outlet openings at the bottoms of the hoppers. For this latter purpose, the hopper outlets are provided with discharge gates or valves.
Government regulations require that the hoppers of hopper dredges be essentially leakproof to prevent water pollution or shoaling by leakage of dredged material during transportation of the material to the off-loading site. Accordingly, it is essential that the hopper discharge gates or valves have effective watertight seals. Prior art hopper dredges have basically two types of hopper bottom closures, viz., generally rectangular hinged doors or cylindrical or conical poppet valve types. These hopper gates undergo severe wear in use, both because of their constant exposure to water and because of the attrition occasioned by the friction of the dredged material passing thereover during dumping of the hopper contents. Furthermore, the dredged material often includes debris which can become jammed in the hopper gates, thereby impairing the operation thereof and preventing watertight closure thereof. Thus, the hopper gates require frequent servicing and repair in order to keep the hoppers leakproof.
In prior art vessels, both the hinged door and poppet valve types of hopper bottom closures are controlled from the decks of the vessels by mechanical linkages extending vertically from the closure member upwardly through the hopper to suitable drive mechanism located on the vessel deck. The hopper closure gates cannot be serviced or repaired in place while the vessel is in service, since normally the gates are submerged under water. Nor can the gates readily be lifted to the decks of the vessel for service because of the mechanical linkage connecting the gates to the equipment on the vessel decks. Thus, by reason of this arrangement, repair or servicing of the hopper closure gates requires dry-docking of the vessel. This constitutes a considerable inconvenience and expense.
Furthermore, since the driving mechanism is situated at deck level above the cargo, the driving force to the gate or valve is transmitted by a long vertical rod which passes through the full depth of the cargo in the hopper. This cargo is usually sand, clay or mud, often having high internal friction. The vibration of the vessel tends to consolidate or "set" the cargo, so that the vertical rod "freezes" because of the high skin friction. In order to start the rod moving through the cargo mass, great force is required to overcome the initial "set" and then to continue to move it despite the skin friction.
The amount of force required is such that the rod must be of large diameter or else requires an annular ring or bearing to brace it midway, thus shortening the free column height. This ring or bearing in turn requires lateral struts to the sides of the hopper. The effect of this structure of bearings and struts is to impede the downward movement of the cargo which is essential to the dumping process. It also results in additional wear and maintenance.
The construction and operation of certain prior art hopper dredges is disclosed in a book prepared by the Office of the Chief of Engineers, U.S. Army, entitled "The Hopper Dredge," United States Government Printing Office, Washington (1954).